Clarify the error in the Inter-Branch merge flow when syntax error

[f-alizada]

• This issue is blocking
• This issue is causing unreasonable pain

Context

There are cases when the the configuration of the merge-flow file is invalid jsonc
The only error we get: WARNING: Failed to fetch and process configuration file and the action completes with success.

Example of such case:
dotnet/msbuild#11190

Suggestion to separate the networking and invalid json format cases for proper reporting.